The issue is stepped-up basis. The term “means that when property passes to an heir, it is assessed at its current value, and taxed at that rate when it is sold, rather than the increase in value from when it was originally acquired by the family,” Abbott explains. Because of large exemptions, “Few farm families pay estate taxes but the 'death tax' is a perennial target of farm groups,” he notes. “There is an $11 million exemption per person from the estate tax, due to a provision in the 2017 tax cut law. The exemption will revert to $5 million in 2026.”
UPDATE, April 28: Stephanie Leiser of the University of Michigan explains capital-gains taxes, in a piece for The Conversation, a site for journalistic writing by academics: "The basis is the original price you paid for the asset. Let’s say you invested $100,000 in some stock and held on to it until you died, at which point it is worth $300,000. If your heirs eventually sell the stock for $700,000, their basis wouldn’t be $100,000 but $300,000, meaning they would pay taxes on only $400,000 in capital gains. But no one will ever pay tax on the $200,000 in appreciation that accrued before you died. Biden’s plan would eliminate this basis step-up and require heirs with incomes over $1 million to pay taxes on the entire amount of their capital gains."